This invention generally relates to method and apparatus for determining the moisture content of materials, and more particularly relates to a dual wavelength infrared technique for measuring moisture content.
As is well known, knowledge about the exact amount of water or moisture present in materials or substances is important for the accurate control of many industrial processes. For example, in the process of making asphalt paving material, sand and gravel, which are collectively referred to as aggregate, are generally mixed with a bituminous or asphalt liquid. However, if the aggregate is not sufficiently hot, the asphalt liquid will not properly adhere to the sand and gravel. Therefore, a large burner is typically used to heat the aggregate before mixing the aggregate with the asphalt liquid. In such process, it is important to know the initial moisture content of the aggregate because that effects how many BTUs are required to heat the aggregate to a sufficient temperature. Therefore, with knowledge regarding the moisture content of the aggregate, the burner can be continuously and accurately regulated. If too little heat is applied, the aggregate will not be sufficiently heated; conversely, if too much heat is applied, the aggregate may be too hot and the energy efficiency of the process will be degraded. In this and other industrial processing systems, the various stages such as the regulation of the burner are controlled by a process computer. In such arrangement, it is desirable to provide the process computer continuously with real time electrical signals representative of the moisture content of the aggregate so that the burner firing rate can be optimized.
Moisture content is generally defined as the ratio of water weight divided by the material weight plus the water weight. One conventional prior art method for determining moisture content is referred to as the water evaporation method. A sample of a material such as aggregate is first weighed, and then it is heated for a sufficient period of time to evaporate or drive off all of the moisture within the sample. Next, the sample is reweighed. The material weight plus water weight is, of course, provided by the initial weighing, and the water weight is the difference between the first weighing and the reweighing after the water has been driven off. One problem with this method is that it requires a substantial amount of time and is labor intensive. Further, the method is not readily adapted to a continuous monitoring system that determines the real time moisture content of a substance such as aggregate immediately prior to a stage where knowledge of moisture content is critical. In fact, aggregate would typically not have a homogenous moisture content, and by the time that one sample on a conveyor belt is analyzed, the aggregate entering the next stage may have an entirely different moisture content.
Another method for determining the moisture content of materials or substances such as aggregate is referred to as the microwave method. Microwave energy is propagated through the aggregate and its conveyor belt, and the theory is that the magnitude of the microwave energy on the other side of the conveyor is a function of the water or moisture content in the aggregate. That is, the more moisture that is present in the aggregate, the more attenuated the microwave energy will be at a detector on the opposite side. This method has a number of apparent disadvantages. First, a relatively high microwave energy power level is required to compensate for energy losses in the conveyor belt and the aggregate itself. Second, the accuracy of the measurement is very limited. That is, it is very difficult to measure small incremental changes in power level caused by absorption by the moisture. Further, the consistency, lossiness and thickness of the aggregate must be very accurately regulated to prevent these factors from becoming variables in the microwave measurements.
Another method of determining moisture content of a sample takes advantage of the fact that infrared energy is known to be absorbed by water at very specific wavelengths. That is, the absorptivity of infrared energy by water or moisture is known to be dependent on wavelength. In one commercially available system, the material, such as aggregate moving on a conveyor belt, is illuminated with broadband infrared energy, and a reflection sensor is positioned immediately above the aggregate. The reflected infrared energy power spectrum is altered according to the amount of moisture on the surface of the aggregate. For example, if the sample has a relatively large amount of moisture on the surface, reflection of energy at wavelengths of high water absorption will be greatly reduced while reflections of energy at wavelengths of low water absorption will be less attenuated by the surface moisture. The spectrum of reflected energy is filtered using wavelength selective optics mounted in a chopper wheel. More particularly, the sensor includes a stationary broadband infrared detector positioned behind a chopper wheel having a plurality of narrow band pass filters each disposed at a different angular orientation around the wheel. Thus, as the wheel rotates, narrow band pass filters of different wavelengths sequentially cover the infrared detector. During a first time period, the infrared detector is exposed to infrared energy of a first wavelength .lambda.1 having a first water absorption characteristic because this is the only light permitted to pass the filter disposed in front of the infrared detector. Then, during a second time period, the infrared detector is exposed to infrared energy of a second wavelength .lambda.2 having a second water absorption characteristic because this is the only light permitted to pass the filter disposed in front of the infrared detector during the second time period. As a result, the detector provides sequential pulses having amplitudes which are a function of the absorption of infrared energy of the respective wavelengths by the surface moisture of the material. For example, the reflection of infrared energy at one of the wavelengths .lambda.1 is not readily absorbed by the surface moisture and thus provides a reference value relating to the surface parameters or characteristics of the material (e.g. how much side reflection there is). The reflection of infrared energy at the other wavelength .lambda.2 is more readily absorbed in surface moisture and thus provides a measure of the surface moisture. By taking the ratio of the pulses for .lambda.1 and .lambda.2, a value proportional to the surface moisture is obtained, and surface moisture generally corresponds to the moisture content of the material. The ratio of .lambda.1 and .lambda.2 can be compared or correlated with data derived from similar measurements previously taken on materials or substances of known moisture content.
The chopper wheel approach, however, has some drawbacks. First, the chopper wheel has to be rotated thereby requiring a drive motor and other associated moving mechanical components that increase the cost and reduce the reliability of the sensor. Also, the transmitted power levels of energy at .lambda.1 and .lambda.2 are relatively weak because they are only a portion of the broadband energy used to illuminate the sample. As such, the ambient light becomes a much more critical factor thus sometimes necessitating the use of shields to shade the sensor and sampled region. It is also apparent that the sensor would have a relatively low signal-to-noise ratio.